1. Field of the Invention
The present invention relates to a fixing device to fix images on recording medium, and an image forming apparatuses incorporating the fixing device.
2. Discussion of the Background
Image forming apparatuses, such as printers, facsimile machines, copiers, plotters, or multi-functional peripherals having several of the foregoing functions, typically include a fixing device that fixes a toner image on a recording medium, such as a paper sheet or overhead projection films (OHP) (hereinafter “sheet”), by heating and pressing the toner image onto the sheet. That is, fixing devices installed in image forming apparatuses execute a fixing process such that heat and pressure are exerted against the sheet to melt the toner, and the toner thus melted is adhered to the sheet.
Such fixing devices include a heating member such as a halogen heater or an induction heating (IH) coil, a fixing member heated by the heating member, and a pressing member that presses against the fixing member with a predetermined pressure. In the fixing process, initially, the heating member heats the fixing member to a predetermined temperature set in advance (hereinafter “fixing temperature”), as part of a reload process (also known as warm-up). Then, after the reload process is finished, the sheet passes between the fixing member and the pressing member, and the image is fixed on the sheet with heat and pressure.
Herein, in the above-described fixing devices, during the fixing process, the fixing member is deprived of a certain amount of heat by the sheet. Therefore, when a great number of sheets are outputted, temperature of the fixing member is decreased. Moreover, although the heating member heats the fixing member to return the temperature of the fixing member to the fixing temperature, the fixing process cannot be started until the fixing member recovers the fixing temperature.
In view of the foregoing, several approaches, described below, have been tried.
In one known method, considering the decrease in the temperature in continuous fixing, the temperature of the fixing member is varied depending on the number of sheets processed. Thus, when the number of sheets is greater, the fixing member is set to a higher temperature to prevent fixing failures. Conversely, when the number of sheets is smaller, the fixing member is set to a lower temperature to reduce a reload time.
However, although the setting temperature of the fixing member is raised when the number of the sheets is greater, the temperature of the fixing member is decreased as the fixing process is performed continuously. Therefore, the amount of heat given to the sheet greatly differs between the initial sheet and the final sheet, and fixing failures, such as unevenness in gloss of images, or unevenness in curl amount of the sheet may occur.
In another known method, pressure between the fixing member and the pressing member is changed based on the sheet size, type of sheet, and the number of sheets output in a continuous fixing process to change the temperature of the fixing member, thereby reducing the reload time while preventing the fixing failure.
However, in this known approach, because the structure of the fixing device is more complicated and the pressure between the fixing member and the pressing member can fluctuate significantly, the image quality, such as image gloss, may fluctuate significantly as well.
Accordingly, there is a need for a technology to improve the fixing reliability and shorten the reload time of the fixing device.